you can load the wav or mp3 with fmod but you’re starting to step outside of what fmod does, this is something for a sound editor, not a playback library.

once you have the 2 loaded sounds, you can get access to their pcm data using Sound::lock / Sound::unlock. Or maybe you could read the data yourself with FMOD_OPENONLY flag and Sound::readData, but you need to mix the data together yourself into a new buffer then write the data to a wav yourself.
fmod does not write wavs or encode mp3s.
Well we do have a wavwriter output but it is a messy way of doing what you want because it still involves playing the sound.